Well known colour video projection systems are comprised of three colour electron guns (red, green and blue) for emitting three separate electron beams which are transmitted through respective CRT face plates having respective red, green and blue phosphor layers. The coloured beams emitted from the posphor on the faceplate of each CRT are then focused together on a projection surface with varying intensities, for creating a composite colour image. Each of the electron beams is magnetically deflected via respective main deflection coils for directing the beams from left to right across the projection surface along a plurality of horizontal scan lines. The number of scan lines appearing on the projection surface for each image (raster) may vary in relation to the scanning frequency of the projection system.
Due to various magnetic influences and non-linearities in the projection system, each of the electron beams may bend more or less in relation to one another at the left and right and top and bottom extremities of the image on the projection surface.
Furthermore, since the respective coloured light beams are projected at different angles for focusing at the centre point, misregistration of the images is known to occur on the edges of the screen, etc.
In order to correct the problem of misregistration of images in projection video systems, convergence systems have been developed for deflecting respective ones of the beams by predetermined amounts in accordance with information input to the convergence system by an operator using a cursor control, etc.
It is known to use vertical and horizontal convergence coils for re-orienting the electron beams at predetermined locations within the image in order to effect proper registration of the three colours.
More recently, automatic convergence systems have been developed which utilize sensors for detecting misregistration of a projected image at various zones on the screen and automatically deflecting the electron beams by the required amounts for obtaining convergence of the image without user input. The sensors in known automatic convergence systems have typically been fabricated using CCD arrays. The CCD arrays detect and digitize the projected image, which is then processed via microprocessor circuitry for detecting misregistration. In response to detecting misregistration, the microprocessor circuitry generates appropriate convergence coil signals, etc. A disadvantage of CCD based sensors is that such sensors integrate the image over an entire raster frame. Accordingly, the signal-to-noise ratio of the detected image relative to detected ambient light is very low. As a result, CCD image sensors are characterized by poor image detection, especially in relation to blue phosphor.